Coronal holes (Altschuler et al., 1972; Munro and Withbroe, 1972) are "dark" coronal regions of the Sun with open magnetic field structure. During the minimum years of the solar cycle they are confined in the Sun's polar regions, while at solar maximum they can be found at all latitudes.

The fast-speed solar wind originates form the coronal holes (e.g., Krieger et al., 1973), and accordingly they are considered the main reason for the "recurrent" type of geomagnetic activity.